Medical brain notes 44

  Adrenocortical Hormones The two  adrenal glands,  each of which weighs about 4 grams, lie at the superior poles of the two kidneys. As shown in Figure 77–1, each gland is composed of two distinct parts, the  adrenal medulla  and the  adrenal cortex.  The adrenal medulla, the central 20per cent of the gland, is functionally related to the sympathetic nervous system; it secretes the hor-mones  epinephrine  and  norepinephrine  in response to sympathetic stimulation. In turn, these hormones cause almost the same effects as direct stimulation of the sympathetic nerves in all parts of the body.         The adrenal cortex secretes an entirely different group of hormones, called  corticosteroids.  These hormones are all synthesized from the steroid cholesterol,and they all have similar chemical formulas. However, slight differences in their molecular structures give them several different but ...

  Synthesis and Secretion of Adrenocortical Hormones The Adrenal Cortex Has Three Distinct Layers.  Figure 77–1 shows that the adrenalcortex is composed of three relatively distinct layers: 1. The  zona glomerulosa,  a thin layer of cells that lies just underneath the capsule, constitutes about 15 per cent of the adrenal cortex. These cells are the only ones in the adrenal gland capable of secreting significant amounts of  aldosterone  because they contain the enzyme  aldosterone synthase,  which is necessary for synthesis of aldosterone. The secretion of these cells is controlled mainly by the extracellular fluid concentrations of  angiotensinII  and  potassium,  both of which stimulate aldosterone secretion. 2. The  zona fasciculata,  the middle and widest layer, constitutes about 75 per cent of the adrenal cortex and secretes the glucocorticoids  cortisol  and  corticosterone,  as well as small ...

  Functions of the Mineralocorticoids- Aldosterone Mineralocorticoid Deficiency Causes Severe Renal Sodium Chloride Wasting and Hyperkalemia.  Total loss of adreno-cortical secretion usually causes death within 3 days to 2 weeks unless the person receives extensive salt therapy or injection of mineralocorticoids. Without mineralocorticoids, potassium ion concen-tration of the extracellular fluid rises markedly, sodium and chloride are rapidly lost from the body, and the total extracellular fluid volume and blood volume become greatly reduced. The person soon develops diminished cardiac output, which progresses to a shocklike state, followed by death. This entire sequence can be prevented by the administration of aldosterone or some other mineralocorticoid. There-fore, the mineralocorticoids are said to be the acute “lifesaving” portion of the adrenocortical hormones. The glucocorticoids are equally necessary, however, allowing the person to resist the destructive effects of li...

  Renal and Circulatory Effects of Aldosterone Aldosterone Increases Renal Tubular Reabsorption of Sodium and Secretion of Potassium.   It will be recalled  that aldosterone  increases  absorption of  sodium and simultaneously  increases  secretion of potassium by the renal tubular epithelial cells especially in the  principal cells of the collecting tubules  and, to a lesser extent, in the distal tubules and col-lecting ducts. Therefore, aldosterone causes sodium to be conserved in the extracellular fluid while increasing potassium excretion in the urine. A high concentration of aldosterone in the plasma can transiently decrease the sodium loss into the urine to as little as a few milliequivalents a day. At the same time, potassium loss into the urine increases several-fold. Therefore, the net effect of excess aldosterone in the plasma is to increase the total quantity of sodium in the extracellular fluid while decreasing the potassium....

  Aldosterone Stimulates Sodium and Potassium Transport in Sweat Glands, Salivary Glands, and Intestinal Epithelial Cells Aldosterone has almost the same effects on sweat glands and salivary glands as it has on the renal tubules. Both these glands form a primary secretion that contains large quantities of sodium chloride, but much of the sodium chloride, on passing through the excretory ducts, is reabsorbed, whereas potassium and bicarbonate ions are secreted. Aldosterone greatly increases the reabsorption of sodium chloride and the secretion of potassium by the ducts. The effect on the sweat glands is important to conserve body salt in hot environments, and the effect on the salivary glands is necessary to conserve salt when excessive quantities of saliva are lost. Aldosterone also greatly enhances sodium absorp-tion by the intestines, especially in the colon, which prevents loss of sodium in the stools. Conversely, in the absence of aldosterone, sodium absorption can be poor, lea...

  Cellular Mechanism of Aldosterone Action Although for many years we have known the overall effects of mineralocorticoids on the body, the basic action of aldosterone on the tubular cells to increase transport of sodium is still not fully understood. However, the cellular sequence of events that leads to increased sodium reabsorption seems to be the following. First, because of its lipid solubility in the cellular membranes, aldosterone diffuses readily to the interior of the tubular epithelial cells. Second, in the cytoplasm of the tubular cells, aldos-terone combines with a highly specific cytoplasmic  receptor protein,  a protein that has a stereomolecularconfiguration that allows only aldosterone or very similar compounds to combine with it. Third, the aldosterone-receptor complex or a product of this complex diffuses into the nucleus, where it may undergo further alterations, finally induc-ing one or more specific portions of the DNA to form one or more types of mes...